make.globals: Set PORTAGE_USERNAME and PORTAGE_GRPNAME variables

[duxsco]

"man 5 make.conf" states that "PORTAGE_USERNAME" and "PORTAGE_GRPNAME" default to "portage":

• portage/man/make.conf.5

  Lines 1257 to 1261 in 597229a

  	\fBPORTAGE_USERNAME\fR = \fI[user]\fR
  	Defines the username to use when executing in userpriv/etc... modes (i.e.
  	non-root).
  	.br
  	Defaults to portage.

• portage/man/make.conf.5

  Lines 1109 to 1113 in 597229a

  	\fBPORTAGE_GRPNAME\fR = \fI[group]\fR
  	Defines the groupname to use when executing in userpriv/etc... modes (i.e.
  	non-root).
  	.br
  	Defaults to portage.

But, the two variables are not set in the files pointed out by "man 5 make.conf":

portage/man/make.conf.5

Lines 21 to 31 in 597229a

	Portage will check the currently\-defined environment variables
	first for any settings. If no environment settings are found,
	Portage then checks the make.conf files. Both /etc/make.conf and
	/etc/portage/make.conf are checked (if present), and settings from
	/etc/portage/make.conf will override settings from /etc/make.conf.
	If no setting is found in the make.conf files, Portage checks
	make.globals. If no
	setting is found there, the profile's default setting is grabbed
	from /etc/portage/make.profile/make.defaults. Please note that all user
	settings should be made in the environment or in the make.conf
	files, which are intended to be customized by the user.

This commits solves the discrepancy.

Bug: https://bugs.gentoo.org/941977

[duxsco]

Testing on my laptop for now.